Assemblies of electrochemical units connected in series (often called stacks) are known. The electrochemical units thus assembled may be formed for example by accumulator elements, or by fuel cells. A fuel cell is an electrochemical device for converting chemical energy directly into electrical energy. For example, one type of fuel cell includes an anode and a cathode between which a proton exchange membrane is arranged, often called a polymer electrolyte membrane. This type of membrane only allows protons to pass between the anode and the cathode of the fuel cell. At the anode, diatomic hydrogen undergoes a reaction to produce H+ ions which will pass through the polymer electrolyte membrane. The electrons generated by this reaction join the cathode by a circuit external to the fuel cell, thus generating an electric current. Because a single fuel cell generally only produces a low voltage (around 1 volt), fuel cells are often series-connected to form fuel cell stacks able to generate a higher voltage which is the sum of the voltages of each cell.
When used within the automobile industry, these fuel cell stacks are associated with a battery to form a hybrid system 100. This system connects the fuel cell stack 102 and the battery in parallel so that the fuel cell stack or the battery 106 simultaneously or separately power the car 108, via a common section called the bus. This hybridization also allows the fuel cell stack to recharge the battery which will supply electrical energy to the car. A hybrid system is called “active” when it uses a DC/DC converter 104 connected to the fuel cell stack output 102 as seen in FIG. 1. This DC/DC converter 104 is used to adapt the voltage levels of fuel cell stack 102 and of battery 106 and to regulate the power delivered by fuel cell stack 102.
Regulating power requires the implementation of a control strategy to distribute the power between fuel cell stack 102 and battery 106 according to the power requirement of the electric engine of the car and system constraints. System constraints which the control strategy has to take into account are the maximum voltages and currents of the fuel cell stack and the battery, the temperature ranges which must not be exceeded, the battery state of charge, i.e. for example, the battery must not be charged when it is already 100% charged, etc.
One of the control strategies for this hybrid system consists in regulating the battery state of charge around a nominal value without ever reaching the maximum or minimum charge of said battery. Thus, the battery never needs to be charged externally, since it is recharged by the fuel cell stack and possibly by recuperating kinetic energy from the vehicle when the latter is in a braking phase. This means that the fuel cell stack supplies the mean power consumed by the electric engine of the vehicle, whereas the battery is used as an energy buffer means for charging or discharging energy. This strategy is implemented by regulating the bus voltage at a constant value using the DC/DC converter.
One drawback of this known strategy is that nothing is implemented to prevent the fuel cell stack from operating at open circuit voltage (“OCV”). “Open circuit voltage” means the area of operation in which the voltage per cell is higher than 0.85-0.9 V/cell. This voltage is known to considerably reduce the lifetime of the fuel cell stack. It is therefore undesirable for the fuel cell stack to operate in this mode. At constant pressure, the fuel cell stack operates in open circuit voltage mode when the load current is small.
The open circuit voltage operating mode may occur when a minimum current is imposed on the fuel cell stack at constant pressure. Indeed, this solution avoids the so-called open circuit mode which occurs when the voltage is higher than 0.85-0.9 V/cell. The voltage increases at constant pressure as the current decreases. The current value determines the power value and it is not always possible to consume the power delivered, particularly if it is no longer possible to charge the battery when its state of charge is close to 100%.
Another case able to cause the fuel cell stack to operate in open circuit mode is when the pressure is reduced. Reducing pressure at low power decreases the cell voltage and thus avoids the open circuit mode. However, it must be considered that pressure variation dynamics are much slower than current variation dynamics and a decrease in pressure can only occur if a current is being consumed. The current value directly affects the pressure reduction speed. Thus, if the fuel cell stack power varies instantaneously from several kilowatts to zero kilowatts, it will not be possible to avoid the open circuit mode, since there will no longer be any current to reduce pressure. Likewise, if the fuel cell stack power has to vary quickly from a power of several watts at low pressure to a power of several kilowatts at higher pressure, the pressure must be increased before the current is increased. This method necessarily causes the fuel cell stack to move, for a short instant, into open circuit mode and thus damages said fuel cell stack.